The major purpose of this project is to study the role of cell surface phenomena in the control of growth, differentiation, and response to external stimuli. We have begun by developing two methods for measuring rates of lateral transport of fluorescent molecules on cell surfaces. Using one method the diffusion coefficients of particles are determined from spontaneous fluctuations of their number in a small open region. In the other method an intense light pulse is used to bleach a small region on the cell surface. Transport coefficients are determined from rates of fluorescence recovery due to entry into the bleached region of fluorophores from unirradiated parts of the membrane. We have used these methods to study the lateral mobility of concanavalin A-receptor complexes, and of proteins nonspecifically labeled with fluorescein and have also compared the mobility of membrane proteins with a lipid probe. During the current year we shall measure the mobility of IgE-Fc receptor complexes on rat peritoneal mast cells and a rat basophilic leukemia cell and the mobility and distribution of acetylcholine receptors on developing rat myotubules. We shall also compare the lateral mobility of membrane constituents of normal and malignant transformed cells. BIBLIOGRAPHIC REFERENCES: J. Schlessinger, D.E. Koppel, D. Axelrod, K. Jacobson, W.W. Webb, and E.L. Elson, Proc. Natl. Acad. Sci. USA, 73, 2409 (1976). "Lateral Transport on Cell Membranes: The Mobility of Concanavalin A Receptors on Myoblasts."